1. Field of the Invention
The present invention generally relates to a motor drive apparatus and a motor driving method. More specifically, the present invention is directed to a speed control technique capable of controlling rotating speeds of a spindle motor, a brushless motor, and the like.
2. Description of the Related Art
Conventionally, to rotate CD-ROMs (Compact Disk-ROMs) and DVDs (Digital Video Disks), for instance, 3-phase spindle motors are known in the field. To drive the 3-phase spindle motors, motor drive apparatuses with employment of PWM (Pulse Width Modulation) control systems are employed. In such a PWM control system, a rotating speed of a motor is controlled by using a pulse width modulated signal. A typical PWM type motor drive apparatus is indicated in FIG. 1. For the sake of a simple explanation, only a circuit arrangement corresponding to a single phase is shown in this drawing. As shown in FIG. 1, this motor drive apparatus is arranged by an energizing (exciting) pulse generating circuit 40, a motor 45, and switches S1, S2, S3 and S4.
The energizing pulse generating circuit 40 is arranged by a logic circuit. This energizing pulse generating circuit 40 generates PWM signals 501, 502, 503, and 504 used to control the rotating speed of the motor 45, and then supplies these PWM signals to the switches S1, S2, S3, and S4, respectively. Each of these PWM signals 501, 502, 503, and 504 is a pulse signal having a time period "T" (will be referred to as a "PWM time period T" hereinafter). A duty factor of each of these PWM signals is varied in response to a rotating speed to be controlled.
The motor 45 has a first input terminal 47 and a second input terminal 48. The rotating speed of this motor 45 is controlled by a first drive signal supplied to the first input terminal 47 and a second drive signal supplied to the second input terminal 48.
The switches S1, S2, S3, and S4 are turned ON when the PWM signals 501, 502, 503, 504 generated from the energizing pulse generating circuit 40 own high levels ("H" levels), and are turned OFF when these PWM signals 501, 502, 503, 504 own low levels ("L" levels).
As indicated in FIG. 1, one terminal of the switch S1 is connected to a VM terminal 46 to which the power supply voltage is applied, and the other terminal thereof is connected to the first input terminal 47 of the motor 45 and one terminal of the switch S2. The other terminal of the switch S2 is grounded. Also, one terminal of the switch S3 is connected to the VM terminal 46, and the other terminal thereof is connected to the second input terminal 48 of the motor 45 and also to one terminal of the switch S4. The other terminal of the switch S4 is grounded.
Operations of the motor drive apparatus with employment of the above-described circuit arrangement will now be described. FIG. 2A shows a waveform (potential level) of the first drive signal supplied to the first input terminal 47 of the motor 45, and FIG. 2B indicates a waveform (potential level) of the second drive signal supplied to the second input terminal 48 of the motor 45. The waveforms of the first drive signal and the second drive signal are determined by turning ON/OFF the switches S1, S2, S3, and S4. In FIG. 2A and FIG. 2B, symbol "T" indicates a PWM time period.
FIG. 3 represents ON/OFF states of the respective switches S1, S2, S3, and S4 in the respective time periods A, B, C, and D of the first drive signal shown in FIG. 2A and of the second drive signal shown in FIG. 2B. FIG. 4A to FIG. 4D indicate paths of motor drive currents flowing from the VM terminal 46 via the motor 45 to the ground point in the respective time periods A, B, C, and D.
When the motor 45 is driven, the energizing pulse generating circuit 40 is first initiated in response to a control signal (not shown) supplied from an external circuit. As a result, the energizing pulse generating circuit 40 outputs the PWM signals 501, 502, 503 and 504 so as to control the turning ON/OFF operations of the switches S1, S2, S3 and S4 every PWM time period in the below-mentioned control manner.
That is, in the time period A of a certain PWM time period, both the switch S1 and the switch S4 are turned ON, and both the switch S2 and the switch S3 are turned OFF. As a consequence, a motor drive current flows from the VM terminal 46 to the ground point via a path indicated by an arrow of FIG. 4A. Accordingly, the motor 45 is driven to thereby rotate along a preselected direction.
Also, in the time period B of this PWM time period, the switch S1 is turned ON, whereas the switch S2, the switch S3 and the switch S4 are turned OFF. As a result, as indicated in FIG. 4B, the current path directed from the VM terminal 46 to the ground point is interrupted, so that the potential at the second input terminal 48 is brought into a non-definition state. Therefore, since no current flows into the motor 45, the motor 45 is not driven. However, the motor 45 itself is continuously rotated due to inertia force.
Subsequently, since the PWM time period is repeated in a similar manner, the motor 45 is rotated along the above-described preselected direction. In this case, the longer the time duration during which the motor drive current flows through the motor 45 is prolonged, namely the longer the time period A in the respective PWM time periods is prolonged, the larger the force produced in the armature of this motor 45 is increased. As a result, the rotating speed of the motor 45 is increased.
When the motor 45 is rotated by a predetermined angle in this manner, since the phase is changed, the motor drive current flowing through the motor 45 must flow through a current path along a direction opposite to the above-described direction. For this purpose, in the time period C of the PWM time period after the phase has been changed, both the switch S1 and the switch S4 are turned OFF, and both the switch S2 and the switch S3 are turned ON. As a consequence, a motor drive current flows from the VM terminal 46 to the ground point via a path indicated by an arrow of FIG. 4C. Accordingly, the motor 45 is driven to thereby rotate along the above-described preselected direction.
Also, in the time period D of this PWM time period, the switch S3 is turned ON, whereas the switch S1, the switch S2 and the switch S4 are turned OFF. As a result, as indicated in FIG. 4D, the current path directed from the VM terminal 46 to the ground point is interrupted, so that the potential at the second input terminal 48 is brought into a non-definition state. Therefore, since no current flows into the motor 45, the motor 45 is not driven. However, the motor 45 itself is continuously rotated due to inertia force.
Subsequently, since the PWM time period is repeated in a similar manner, the motor 45 is rotated along the above-described preselected direction. In this case, similarly, the longer the time duration during which the motor drive current flows through the motor 45 is prolonged, namely the longer the time period C in the respective PWM time periods is prolonged, the larger the force produced in the armature of this motor 45 is increased. As a result, the rotating speed of the motor 45 is increased.
In the above-explained conventional motor drive apparatus, the rotating speed of the motor 45 is increased in the case that the duty factor of the PWM signal is changed so as to prolong the ON-time of the switches S1 and S4 in the time period A, and furthermore the duty factor of the PWM signal is varied in order to prolong the ON-time of the switches S2 and S3 in the time period C. As a result, the better acceleration following characteristic can be obtained.
However, in this conventional motor drive apparatus, there is a problem that the speed-reduction following characteristic cannot be obtained under better condition. In other words, to reduce the motor rotating speed, the energizing pulse generating circuit 40 changes the duty factor of the PWM signal in such a manner that the ON-time of the respective switches S1 and S4 in the time period A and the respective switch S2 and S3 in the time period C are shortened. However, in the rear half of the PWM time period (namely, time period B and time period D), all of the current paths for the motor drive currents flowing through the armature of the motor 45 are interrupted. As a result, the motor is continuously rotated due to inertia force without receiving any braking force. As a consequence, the speed-reduction following characteristic of the motor 45 is considerably deteriorated.
Also, for example, spindle motors of CD-ROM drives require very high speed-reduction following characteristics. To meet such a requirement, a speed reducing means for executing either short braking operation or reverse rotating operation may be provided in order to reduce the rotating speed of the motor continuously rotated due to the inertia force. However, there is another problem that when such a speed reducing means is separately provided, the complex rotation speed control sequence for the spindle motors is necessarily required.
The related art is disclosed in Japanese Patent Laid-open Disclosure (JP-A-Heisei 5-211780) filed in the priority basis of U.S. Pat. No. 5,309,078 entitled as "SYNCHRONOUS RECTIFICATION METHOD FOR REDUCING POWER DISSIPATION IN MOTOR DRIVERS IN PWM MODE". That is, this synchronous rectification method is related to Pulse Width Modulation (PWM) techniques, often used to reduce the power dissipation in polyphase motors, chop the current in the coils of the motor at their peak current levels, to achieve maximum torque, to allow rapid accelerations, and to reduce the power dissipated in the chip to a level proportional to the duty cycle. During the time the current is switched OFF, the current which has been established in the coils of the motor is allowed to be dissipated. Accordingly, when the switching transistors of the active coils are turned OFF during PWM mode chopping a non-rectifying ground return path is provided by switching transistors in driving coil. This non-rectifying ground return path is provided by switching transistors in parallel with flyback diodes, operated in a form of synchronous rectification. This gives an alternate current path for the coil current to reduce the voltage drop across the diodes, and thereby reduce the power dissipation and heat in the chip.
However, this synchronous rectification method described in JP-A-Heisei 5-211780 does not open the technique capable of highly improving the speed-reduction following characteristic.